The recording density of a hard disk drive (HDD) as a kind of a magnetic recording/reproducing apparatus is presently increasing 50% or more per year, and this tendency presumably continues in the future. Accordingly, the development of a magnetic head and magnetic recording medium suitable for a high recording density is being made.
To increase the recording capacity of a presently commercially available magnetic recording/reproducing apparatus, the recording density of a magnetic recording medium is more and more increased. If the recording density is kept increased in the future, this may pose the problem of a thermal decay limit caused by downsizing of magnetic particles of a magnetic recording medium, and the problem of a high-density limit of the existing recording method caused by deterioration of the recording capability resulting from narrowed tracks of a magnetic head. As recording methods of breaking through this recording density limit, assisted recording methods such as thermally assisted recording and high frequency assisted recording are attracting attention.
The high frequency assisted recording method uses the ferromagnetic resonance of a thin magnetic film. That is, a recording head includes a high frequency oscillator called a spin torque oscillator, and a magnetic recording medium is irradiated with a high frequency magnetic field generated around this high frequency oscillator. If the high frequency magnetic field has a frequency band close to the magnetic resonance frequency of the magnetic recording medium, the medium magnetization resonates and readily reverses. Therefore, a recording magnetic pole is formed near the high frequency oscillator in the recording head. Consequently, even when a DC magnetic field applied from the recording magnetic pole to the medium is not so large compared to the medium coercive force, the recording magnetization direction of the medium magnetization can be matched with the recording magnetic pole direction. This facilitates information recording to the medium magnetization.
Several arrangements of the high frequency oscillator for use in this high frequency assisted recording have been proposed. For example, an oscillator using a magnetic layer having negative anisotropy as a magnetization oscillation layer has been proposed. In this oscillator, a spin injection layer having magnetic anisotropy perpendicular to the film surface and the magnetization oscillation layer having negative anisotropy are stacked with a nonmagnetic interlayer being sandwiched between them. In addition, electrodes for supplying an electric current in a direction perpendicular to the film surface of this multilayered magnetic film are formed at the two ends of the multilayered film. When an electric current is supplied to the oscillator formed by this multilayered magnetic film, electrons transmitted through or reflected by the spin injection layer are spin-polarized to one polarity in accordance with the polarity of magnetization in the spin injection layer, and flow into the magnetization oscillation layer having negative anisotropy through the nonmagnetic interlayer.
This polarized electron spin generates a spin torque force that rotates the magnetization of the magnetization oscillation layer in a predetermined direction. This spin torque force keeps rotating the magnetization oscillation layer in a predetermined direction in the film surface. Since the cycle of this rotation is generally a few GHz to a few ten GHz, a high frequency oscillator having a frequency band from a few GHz to a few ten GHz can be implemented.
The oscillation frequency of this oscillator is determined by, e.g., a damping constant α and magnetic anisotropy Ku of the thin magnetic film forming the magnetization oscillation layer, and the density of an electric current supplied to the oscillator. As the magnetic anisotropy of the magnetization oscillation layer increases in the negative direction, and as the current density increases, the oscillation frequency of the oscillator increases.
When designing a high frequency oscillator for use in high frequency assisted recording, the oscillation frequency must be brought near the magnetic resonance frequency of a magnetic recording medium. Generally, the magnetic resonance frequency of a magnetic recording medium having a high positive anisotropic magnetic field of Hk=15 kOe or more is 20 GHz or more. Therefore, the oscillation frequency of the high frequency oscillator must also be set at 20 GHz or more. However, in a conventional magnetic material having a large negative anisotropy, the anisotropy is at most about Ku=−4×106 erg/cc (−4×108 nJ/cc), and this negative anisotropy is too weak to apply the material to the high frequency oscillator as described above.
The method of raising the oscillation frequency of the high frequency oscillator as described previously can also be implemented by raising the density of an electric current to be supplied to the oscillator, instead of increasing the negative anisotropy of the material. Since, however, the oscillator is mainly formed by the multilayered thin magnetic film, the oscillator melts due to heat if the current density is raised too much. This makes it difficult to realize a current density equal to or lower than a predetermined limit. Under the circumstances, it is desirable to form a thin magnetic film having a stronger negative anisotropy in order to implement a high frequency oscillator for use in high frequency assisted recording.